If a question is really no longer relevant, then presumably it would not be very likely to receive new answers, making closing a sort of redundant concept. On the off chance that someone's circumstances led them to a situation where the answers such a question received were relevant, then there's no harm in leaving it around.
In certain cases, stale questions that lose relevance can become actively harmful by drawing people to the wrong information, at which point we have a couple of options:
The question can be closed as a duplicate of the question that's actually helpful in a similar search
The question can be deleted if it's causing people to trip over it, or advertising incorrect information or guidance.
The idea of putting posts on hold, later changing to a closed state is so that they can be actively improved so that they're suitable for reopening. If something is gathering dust and making anyone happening by it sneeze, then it's probably something that should just be deleted.
If you think something is actively harmful and should be deleted without hesitation or ceremony, then you can flag it as 'other' for moderator attention and let them know why it might need to be removed. Just be as specific as you can, include any informative links, give the moderators the same background that you have on the subject.
If it's not harming anything, then there's really no reason to think much more about it.